1. Field of the Invention
The present invention is related to a pulse amplitude modulation method for the direct current (DC) brushless motor without using sensor device, and more particularly, related to a pulse amplitude modulation method utilizing a motor phase rotation driving device to control the signal to drive the motor driving circuit so as to avoid the motor driving circuit shutdown too early, and the motor driving circuit can be quickly recharged at the next startup period to achieve the power saving and the fart charging purpose.
2. Description of the Prior Art
The technique related to the DC brushless motor in prior art discloses a anti-noise method utilizing a startup circuit to output different startup frequencies and output different driving currents from the control circuit to pass through the driving coil of the external motor and feedback the external BEMF to detect by the detective circuit so as to determine the startup and the operation of the motor in order to guarantee the motor is working properly.
As shown in FIG. 1A, it is a block diagram illustrating that a DC brushless motor system without sensor device. As shown in FIG. 1A, the system includes an external motor 11, a control circuit 12, a output circuit 13, a detective circuit 14, a startup circuit 15, and a switching circuit 17. The startup circuit 15 outputs different driving frequencies square waves to the output circuit 13 and the corresponding output current is outputted to the driving coil of the external motor 11. The driving coil of the external motor 11 will generate the Back Electromotive Force (BEMF) to feedback to the detective circuit 14 and the detective circuit 14 will determine the rotation speed and the phase of the external motor 11 in accordance with the BEMF so as to control the startup and the rotation speed of the motor.
FIG. 1B is a view illustrating the conventional six-step motor driving circuit in the DC brushless motor. As shown in FIG. 1B, when the startup circuit 15 is activated to output the activated frequency signal to the control circuit 12, the signal is transformed to be a six steps driving control signal shown in FIG. 1B, into the output circuit 13. The current of the driving coil of the external motor 11 is accordance with the phase difference of the six steps driving control signal, and the rotation speed and the phase of the external motor 11 is determined by the current of the driving coil.
FIG. 2 is a view illustrating a conventional pulse amplitude modulation (PAM) circuit in prior art. As shown in FIG. 2, the PAM circuit 20 includes a first input transistor 201, a second input transistor 202, an internal resistor 203 and an output capacitance 204. When the pulse width modulation (PWM) control signal is inputted into the first input transistor 201 of the PAM circuit and the PWM control signal is in high voltage level, the first input transistor 201 is turned on and a current outputted from the voltage Vcc transmits to the internal resistor 203. A voltage drop is generated on the internal resistor 203 to turn on the second input transistor 202 and the current will transmit to the second input transistor 202 from the Vcc and the output capacitance will start to charge. When the PWM control signal is in low voltage level, the first input transistor 201 is turned off and a current outputted from the voltage Vcc stops transmitting to the internal resistor 203. The voltage drop is zero and the second input transistor 202 is still off and the output capacitance will start to discharge when the voltage is in low voltage. Still referring to FIG. 2, it is illustrating the input and the output of the PAM circuit. When the PWM control signal of the PAM circuit is in high voltage level, the PAM signal is the positive slope charge voltage. When the PWM control signal of the PAM circuit is in low voltage level, the PAM signal is the negative charge voltage.
As the description above, the PAM circuit 20 is configured to convert the PWM control signal to be the PAM driving signal so as to provide the entire driving system power. However, when the motor is rotated, the current is consumed to accelerate the discharge speed of the output capacitance. When the PWM control signal is not returned high voltage level, the output capacitance will keep discharging to a low working voltage of the system and the entire driving system would not work properly and the phase of the motor is required to reset and the pulse width of the PWM is limited.
As the PAM circuit described above, a detective circuit is disclosed in the present invention to detect in accordance with the PAM control signal. When a low voltage level is detected, the driving circuit is shutdown immediately to slow the discharging time to avoid the motor control phase circuit shutdown too early. Therefore, the motor six step driving voltage is outputted properly in the next charging period to extend the time of the low voltage level of the PWM so as to achieve the low rotation speed control and the power saving.